Because of their low cost, small mass, low signal attenuation over long lengths, immunity to electromagnetic interference and potential for high bandwidth data transfer, optical devices (e.g., fibers, wafers) have been developed for sensing applications. Optically based sensors may be used for measuring, for example, temperature, strain, pressure and acceleration with applications varying from smart structures, to the oil and gas industry, to the life science industry. Intrinsic optical sensors use the optical fiber itself as the sensing element. Examples of intrinsic optically based sensors are fiber Bragg grating sensors and intrinsic Fabry-Perot interferometric sensors. These sensors function well for single mode fiber, but function less well, or not at all, if the fiber is multimodal. Single crystal sapphire optical fiber can potentially operate at higher temperatures than silica optical fiber. However, whereas silica optical fiber can be manufactured such that it propagates a single optical mode, single crystal sapphire optical fiber is highly multimodal, due to its relatively large diameter and the large change in the index of refraction in passing from the sapphire fiber to the air that surrounds the fiber, when the fiber is used in air.
Optical fiber-based sensors are capable of distributed temperature and/or strain measurements with high accuracy and sub-cm spatial resolution. Optical fibers have a small diameter (˜100 μm), a small mass (and therefore fast response time), and are fabricated from high temperature tolerant, radiation-hard materials. Optical fiber sensing is already used in the petroleum industry to measure the temperature and pressure profiles in down-hole applications. Other harsh environments in which optical fiber sensing is used include coal gasifiers and jet engines. The Nuclear Regulatory Commission has shown interest in using fiber optics for advanced instrumentation in high temperature next generation nuclear power plants. Specific applications include monitoring fuel performance during irradiation in test reactors, or embedding fibers in commercial reactor components or structures. Single crystal sapphire (e.g., α-Al2O3) optical fiber has a melting temperature in excess of 2000° C. This is ideal for extremely high temperature sensing of, for example, fuel centerline temperature, during irradiation testing of metallic fuels and some oxide fuels. Unfortunately, sapphire optical fiber exhibits highly multimodal light transmission and is therefore unsuitable for most optical sensing techniques.
There are currently four different sensing techniques that can be applied to optical fibers to produce distributed sensing along the fiber (Optical Frequency Domain Reflectometry, Optical Time Domain Reflectometry (OTDR), Stimulated Brilloiun Scattering and Bragg Grating Sensing). With the exception of Raman-based distributed sensing, a type of OTDR sensing that has been demonstrated in sapphire fiber to temperatures up to 1200° C., most sensing techniques require nearly single mode light transmission in an optical fiber because the measurement is made by either interferometric or time of flight methods. For both methods, the measurements can be distorted by additional light modes in the fiber.
Optical Frequency Domain Reflectometry (OFDR) is a distributed measurement technique that works on the principles of injecting light into an optical fiber and measuring the reflected light off reflection points caused by natural defects or intentionally inscribed defects in the fiber. First, a ‘map’ of the reflection points within the fiber is made at a known temperature (usually room temperature). As temperature or stress causes the fiber to expand or contract, the location of these reflection points moves. The measurement of their movement can be correlated to temperature or strain. The OFDR technique has been successfully used to measure temperature or strain with silica optical fibers.
The Optical Backscatter Reflectometer (OBR) OBR4600 produced by Luna Innovations uses the OFDR technique to interrogate optical fibers and is the instrument that was used for this work. The OFDR technique only works if the fiber is nearly single mode. If the optical fiber supports multiple light modes, the time of flight measurement will be distorted relative to the time of flight for the primary mode, because the light can travel along different path lengths in the fiber. Luna Innovations has used multimode fiber for temperature and strain measurements; but to do so, the multimode fiber had to be aligned perfectly with a single mode fiber, so that only a few light modes would be injected into the multimode fiber. Also, the multimode fiber could not be bent. Presently, only silica optical fibers are used with the OFDR technique, because they can be made to support only a single light mode and because they have inherent reflection points within them that are due to defects and density changes of the amorphous silica glass structure. Sapphire fiber's multimodal nature, due to its large core and the lack of a cladding, along with the deficiency of defects in sapphire, due to its crystalline structure, makes the interference based OFDR sensing technique fail with normal sapphire fiber.
Silica glass based optical fibers have a maximum temperature range of approximately 800° C., due to the limitations of the silica glass (transmission through the glass decreases and reflection points evolve dramatically around that temperature). Single crystal sapphire optical fibers transmit light at temperatures well above 800° C. They would potentially provide a sufficiently high operating temperature for temperature measurements to be made for fuel pins and in other high temperature regions in a reactor using OFDR, if sapphire fiber could be made sufficiently single mode in its transmission characteristics.